Method of controlling a diode device for use in optical storage systems

ABSTRACT

A system and method is provided which compensates for the effects of relaxation oscillations and turn-on delays of diode laser devices. In particular, there is provided a method and system for tuning the shape of the power profile of an output optical signal and its position with respect to a channel bit clock of an optical recording system.

The present invention relates generally to the field of optical storagesystems, and more specifically to the positioning and shaping of anoutput optical signal in an optical storage system.

A semiconductor laser, or ‘diode laser device’ generally consists of anoptically active layer, sandwiched between p-type and n-type dopedsemiconductor materials. When forward biased, electrons (or ‘carriers’)and holes accumulate in the active region of these so-calledhetero-junction devices where they can recombine to emit a photon. Atypical diode laser device 1 formed on a semiconductor substrate 3 isshown in FIG. 1. An active layer 5 (formed from GaAs, for example) issandwiched between a p-layer 7 (p-GaAlAs for example) and an n-layer 9(n-GaAlAs for example). Metal contacts 11 and 13 facilitate theintroduction of a current flow into the device 1. Light formed byrecombination of electrons and holes from the n- and p-layersrespectively is emitted by the device 1 out of the active layer 5. Theends 15,17 of the semiconductor device 1 are usually polished, orcleaved, in order to form an optical resonant cavity in which someemitted photons are retained within the cavity. The retained photonsresonate in this resonant cavity defined by the ends 15,17 of the device1, one of which (17 in this particular example) is generally onlypartially reflective. The end 17 may be a half silvered mirror forexample. The photons that remain within the cavity stimulate emission offurther photons within the active layer 5. Due to the partiallyreflective nature of the end 17 of the device, a small percentage of thephotons emerge from the device 1 forming an intense, coherent (i.e.spatially and temporally in phase) optical signal. When the device 1 isoperating in this regime it is said to be ‘lasing’, and the opticalsignal is typically a short burst or pulse of light.

The typical optical transient behaviour of such diode laser devices maybe modelled using rate equations that describe the rate of change ofcarrier and photon densities. The use of such rate equations enableslaser devices to be modelled and tested theoretically before fabricationfor verification and experimental purposes.

One of the characteristics of diode laser devices which is a veryimportant factor to be considered when such devices are used forapplications in which a prompt lasing response to an input current isrequired, is the so-called ‘turn-on delay’. If the diode laser device isbiased below a threshold current (the current at which laser operationoccurs), only spontaneous emission of photons occurs, and no laseroperation is observed. Some photons may escape from the device in thissituation due to spontaneous recombination of electrons and holes, butthe amount of light produced by the device in this case will benegligible compared to the situation in which the laser is operating viastimulated emission. In addition, the light emerging as a result ofspontaneous recombination processes will generally not be coherent.

As is well known and understood, as soon as the bias current is raisedabove the threshold value however, extra carriers are injected into theactive layer of the device until the threshold condition for lasing isreached. At that point an inversion in the population of carriersoccupying the upper and lower energy levels of the laser medium occurssuch that a substantial percentage of atoms in the laser medium areexcited into an upper energy state. It is the finite time before thecondition for lasing is reached that is the cause of the turn-on delay.Since the present invention is concerned with this turn-on delay, theoperation of the diode laser device will not be described in any greaterdetail.

Therefore, the turn-on delay occurs as soon as the input current hasbeen switched on as it takes some time τ_(on) before the laser actuallystarts lasing.

In an optical storage system where a diode laser device is used torecord data onto an optical storage medium, a delay in writing data dueto the effects of turn-on delay can cause data to be misrepresented onthe medium, and this can lead to errors when the data is read back. Inextreme cases such errors may lead to a failure to read the opticalmedium altogether. A basic schematic diagram of an optical storagesystem 100 is depicted in FIG. 2 of the accompanying drawings. Acontroller 101 is operable to manage operation of a diode laser device103. Specifically, the controller 101 provides the laser device 103 witha control signal which serves to initiate an output optical signal 105from the device 103 at a given time. The optical signal 105 is incidenton an optical storage medium 107 in order to write data to the medium107 in accordance with standard write procedures. The optical signal 105is typically of the form of a short burst or pulse of light.

The optical storage system 100 may operate by applying the opticalsignal 105 to the optical storage medium 107 for a specified length oftime whenever a clock signal of the system occurs, in order to altersome characteristic of the storage medium 107 such as reflectivity, forexample. The controller 101 may therefore provide the laser device 103with a control signal whenever a clock signal is received. If, however,the laser device 103 does not produce an output optical signal for atime τ_(on) after the clock signal then the altered characteristic willnot occur at the expected position on the storage medium. This canresult in the problems mentioned above.

FIG. 3 shows the effects of turn-on delay in relation to the outputpower of a typical infrared laser diode device. As shown, and asdescribed above the diode laser takes a time, τ_(on), before lasing, andoutput of the optical signal, occurs. There is also an appreciable‘relaxation oscillation’ which occurs as a result of the complexinterplay between filling and emptying the carrier and photon reservoirsof the laser diode device, and which causes fluctuations in the outputpower of the optical signal near the beginning of the signal. Ideally,the output power profile of the output optical signal is square inshape, particularly for optical systems of the type described above. Asquare profile enables data to be written to an optical storage mediumuniformly. It is clear, therefore, that the relaxation oscillations andturn-on delay cause the tuning and power profile of the output opticalsignal to be less than optimal.

Accordingly, both effects have a considerable effect on the reliabilityand effectiveness of write strategies in optical recording systems. Thisis especially apparent with high bit rates (for example, a contemporaryDVD recorder may a have a bit rate of 250 Mb/s) since errors will occurat a higher rate.

U.S. Pat. No. 5,831,959 to Mitsumi Electric Co. describes a lasercontrol circuit suitable for use in a recordable optical disk drive. Anauxiliary current is fed into a bypass capacitor so that the chargecurrent for the bypass capacitor is increased. In this connection, thetime to start the emission of a laser beam used for recording isreduced. U.S. Pat. No. 5,831,959 does not solve the problems ofprecisely positioning and shaping the optical signal with respect to theclock signal of the system however, and is only directed towards a basicreduction in turn-on delay. Specifically, U.S. Pat. No. 5,831,959 doesnot enable the power output profile of an optical signal to be tuned,nor does it solve the problems associated with unwanted oscillations(relaxations oscillations) at the start of an optical signal as solvedby the present invention and as described below.

There is therefore a need for a method and system which is able tocompensate for the effects of relaxation oscillations and turn-on delaysof diode laser devices, especially when such diode laser devices areused in optical storage systems in which such effects are highlyundesirable for the reasons outlined above.

In particular, a method and system is required in which the profile ofthe output power of an optical signal and the position of an opticalpulse associated with the optical signal may be tuned, so that theoptical pulse of an optical recording system may be synchronised withthe channel bit clock of the system as required.

According to a first aspect of the present invention there is provided amethod of controlling a diode laser device which is operable to receivea control signal and to output an optical signal when the control signalexceeds a threshold value, the method comprising: supplying, to thediode laser device as the control signal and at a predetermined turn-ontime, a bias signal having a value which exceeds the threshold value,

characterised by supplying to the diode laser device, as the controlsignal and at a predefined time before the predetermined turn-on time, apre-bias signal, which has a magnitude less than the threshold value andextends for a time period, the predefined time, magnitude, and timeperiod of the pre-bias signal determining a required output powerprofile of the output optical signal.

According to a second as aspect of the present invention there isprovided a method of controlling a diode laser device in an opticalsystem, the system including a laser diode device a controller, whereinthe laser diode device is operable to receive a control signal from thecontroller and to output an optical signal when the control signalexceeds a threshold value, the method comprising:

supplying, to the diode laser device as the control signal and at apredetermined turn-on time,

a bias signal having a value which exceeds the threshold value,

characterised by supplying to the diode laser device, as the controlsignal and at a predefined time before the predetermined turn-on time, apre-bias signal, which has a magnitude less than the threshold value andextends for a time period, the predefined time, magnitude, and timeperiod of the pre-bias signal determining a required output powerprofile of the output optical signal.

According to a third aspect of the present invention there is providedan optical system comprising,

a controller operable to output a control signal; and

a laser diode device operable to receive a control signal from thecontroller, and to output an optical signal when the control signalexceeds a threshold value, wherein the controller is operable to outputto the laser diode device, as the control signal and at a predeterminedturn-on time, a bias signal having a value which exceeds the thresholdvalue to the laser diode device,

characterised in that the controller is operable to output to the laserdiode device, as the control signal and before the predetermined turn-ontime, a pre-bias signal to the laser diode device, which pre-bias signalhas a magnitude less than the threshold value and extends for a timeperiod, the predefined time, magnitude, and time period of the pre-biassignal determining a required output power profile of the output opticalsignal.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description ofembodiments thereof, taken in conjunction with the accompanying drawingsin which

FIG. 1 is schematic representation of a typical semiconductor diodelaser device;

FIG. 2 is a schematic representation of a previously considered opticalstorage system;

FIG. 3 is a set of two graphs showing how output power and carrierdensity of a typical InGaAsP infrared diode laser device varies inrelation to input current;

FIG. 4 is a schematic representation of an optical storage system inaccordance with an aspect of the present invention;

FIG. 5 is a set of two graphs showing how output power and carrierdensity of a typical InGaAsP infrared diode laser device varies inrelation to input current according to an aspect of the presentinvention;

FIG. 6 is a set of two graphs showing how output power and carrierdensity of a typical InGaAsP infrared diode laser device varies inrelation to input current according to an aspect of the presentinvention;

FIGS. 7 a and 7 b show a set of four graphs showing how output power andcarrier density of a typical InGaAsP infrared diode laser device variesin relation to input current according to an aspect of the presentinvention; and

FIG. 8 is a set of two graphs showing how output power and carrierdensity of a typical InGaAsP infrared diode laser device varies inrelation to input current according to an aspect of the presentinvention.

FIG. 4 is a schematic representation of an optical storage system 200 inaccordance with one aspect of the present invention.

A controller 201 is operable to manage operation of a diode laser device203. Specifically, the controller 201 provides the laser device 203 witha control signal which serves to initiate output of an optical signal205 from the device 203 at a given time. The optical signal 205 isincident on an optical storage medium 207 in order to write data to themedium 207 in accordance with standard write procedures. The opticalsignal is typically of the form of a short burst or pulse of light.

The optical storage system 200 may operate by applying the opticalsignal 205 to the optical storage medium 207 for a specified length oftime whenever a clock signal of the system occurs in order to alter somecharacteristic of the storage medium 207 such as reflectivity, forexample. The controller 201 may therefore provide the diode laser device203 with a control signal whenever a clock signal is received.

A typical control signal 202 of the controller 201 is shown at 202 ofFIG. 4. The control signal 202 is a step function, with its lower valuebelow a value ‘T’ but greater than zero, and its higher value above avalue ‘T’. The higher value occurs at a later time than the lower value.The value ‘T’ is the threshold value at which the laser device begins toexhibit lasing operation. Hence, according to the system of FIG. 4, thelaser device 203 is supplied with a control signal which is pre-biasedat a value below threshold. The pre-bias signal will advantageously beapplied to the laser device 203 before a clock signal of the system 200.The higher value of the control signal will occur simultaneously withthe clock signal in order to provide the output 205.

FIG. 5 shows a set of two graphs depicting how output power and carrierdensity of a typical InGaAsP infrared diode laser varies in relation tothe input current according to an aspect of the present invention.Although the present invention will be described with reference totypical InGaAsP infrared diode laser devices, it will be appreciated bythose skilled in the art that the method of the present invention isapplicable in general to all diode laser devices which require an inputcurrent above a certain threshold value in order to achieve a lasingcondition.

As shown in FIG. 5, the input current is stepped up to an initial levelwhich is slightly below the threshold current value required for lasingto occur. In comparison with the case shown in FIG. 3, it is clear thatthis pre-biasing of the input current has significantly reduced theturn-on delay. By modifying the amplitude of the pre-biasing signal, thepower profile of the output optical signal can be tuned. The amplitudemodification affects the interplay between the generation and emissionof photons and the recombination of charge carriers such that theemitted optical signal has a significantly squarer power profile.

Therefore, by adjusting the value (i.e. magnitude and/or duration) ofthe pre-bias signal, the output power profile of the optical signal maybe finely tuned to the desired shape. Specifically, and as shown in FIG.5, the relaxation oscillations may be attenuated to the extent that theprofile tends to the desired square shape. As mentioned, a square powerprofile of the output optical signal is desirable as it enables data tobe written to an optical recording medium with greater accuracy, andwith greater uniformity when compared to optical systems in which astandard diode laser system is used.

The present invention therefore provides significant improvement overprior systems, as the output power profile of the optical signal may befinely tuned to provide the desired shape and timing characteristics.

FIG. 6 shows two graphs depicting how output power and carrier densityof a typical InGaAsP infrared laser diode varies in relation to theinput current according to a second aspect of the present invention.FIG. 6 shows that the input pre-biasing current has been adjusted invalue from that shown in FIG. 5. Specifically in this exemplaryembodiment, the pre-biasing signal has a longer duration, and a reducedheight (i.e. current value) compared to the pre-biasing signal of FIG.5.

As can be seen, this results in an output optical signal withsignificantly reduced turn-on delay compared to the prior art systemsshown in FIG. 3. In addition, the shape of the optical signal has beenaltered by the tuning of the pre-biasing current. Specifically, therelaxation oscillations have been significantly reduced resulting in anoutput optical signal with a much squarer power profile than previouslyobtained and as shown in both FIGS. 3 and 5.

As before, this is particularly desirable in optical recording systemsas it means that there is less variation in the output power of theoptical signal near the start of the signal, meaning that informationmay be written in a more uniform fashion onto an optical storage mediumsuch as compact disc-recordable media (e.g. CD-R, CD±RW), DVD-recordablemedia (e.g. DVD-R, DVD+RW), or Blu-Ray discs etc. The application of thepre-bias signal as depicted in FIG. 6 illustrates that the output powerprofile of the output optical signal may, advantageously, have its shapefinely tuned to suit a particular application. In this connection, thepower profile of FIG. 6 is significantly square in shape which isparticularly advantageous for optical recording systems as explainedabove.

It will, however, be appreciated by those skilled in the art, that theexact nature (i.e. duration and/or magnitude) of the pre-bias pulse (orpulses) may be adjusted as necessary in order to obtain the requiredoutput power profile of the output optical signal, and that the examplesgiven herein are presented merely as an aid in order to explain theinventive concept embodied by the present invention.

For example, the pre-bias signal may comprise of a multitude of‘pre-pulses’ arranged in a step formation. Alternatively, the pre-biassignal may comprise of a combination of pre-pulses having temporallyvarying magnitudes (such as triangular shaped pre-pulses for example, orcurved pre-pulses). The exact nature of the pre-bias signal, and inparticular its magnitude at a given time and its overall duration may,therefore, be determined for the particular application in order tofinely tune the output power profile of the output optical signal tothat desired. It is not, therefore, intended that the pre-bias signalsbe limited to those given in the examples and shown in the accompanyingfigures.

FIG. 7 shows a further example for a specific laser device withthreshold current of 46 mA operated with a current peak value of 100 mA.In FIG. 7 a the laser is biased with 0 mA current, whereas in FIG. 7 bthe bias current is just above threshold. As can be seen from thefigures, the turn-on delay for this device is 0.34 nsec and 0.11 nsecfor operating conditions I_(bias)=0 mA and I_(bias)=50 mA respectively.As expected it takes a longer time for the laser device to start lasingwhen the carrier reservoir has to filled from empty, i.e. when thecurrent bias equals zero or almost zero (see FIG. 7 a). Therefore,according to the present invention, the turn-on delay can be minimizedby keeping the bias current between two succeeding optical pulses at anacceptable level near threshold. Since the trailing edge of the opticalpulses is at the same position for both situations (there is no suchthing as “turn-off” delay), this implies that the exact nature of thepower profile of optical signal depends on the bias conditions.

This is of crucial importance in high speed phase change recording,where the physical length of the written marks on an optical recordingmedium is directly related to the optical pulse duration. A deviation ofthe pulse length from an integer multiple of the bit clock yields timingjitter, and hence leads to degraded read-out performance.

FIG. 7 demonstrates that, according to the present invention theposition of the output optical signal can be exactly tuned to coincidewith the (current) bit clock by applying a pre-bias signal just beforethe actual writing pulses of the output optical signal. This pre-biassignal brings the carrier density just below its threshold value at thetime when the actual code word is going to be written and the diodelaser device should start to emit photons.

One can also see that the shape of the leading edge of the opticalsignal, normally dominated by relaxation oscillations, can be tuned veryaccurately such that the ringing disappears completely. This opticalsignal shaping requires additional write levels in phase-change writestrategies that can be accurately controlled in time domain(sub-nanosec). A very sharp transition can therefore be obtained usingthe method of the present invention, regardless of the internalelectrical impedance of the laser device.

FIG. 8 shows how output power and carrier density of a typical InGaAsPinfrared diode laser device varies in relation to the input currentaccording to a fourth aspect of the present invention. As can be seenthe pre-bias signal has been stepped several times up to the valuerequired above the threshold current value.

By stepping the current in this manner, the shape of the output opticalsignal can be accurately tuned as shown in FIG. 8. Specifically, animproved square shape of the output optical signal power profile isobtained with virtually no ringing (relaxation oscillations). Again, theturn-on delay has been effectively reduced to zero.

Although the present invention has been described with reference tooptical storage systems, it will be appreciated by those skilled in theart that the present invention is applicable to all situations in whicha prompt, well tuned lasing response to an input current is required. Inparticular for example, the present invention may advantageously beapplied to medical laser uses in which it is desirable to have a prompt,highly tuned output optical signal with no fluctuations in output power(e.g. ringing).

It will further be appreciated that the exact shape of the output powerprofile of the output optical signal as described herein and as shown inthe accompanying figures is not intended to be limiting. In particular,it is intended that the shape of the output power profile of the outputoptical signal is capable of being tuned to suit a particularapplication, and need not, therefore, necessarily be square in shape.

1. A method of controlling a diode laser device (203) which is operableto receive a control signal (202) and to output an optical signal (205)when the control signal (202) exceeds a threshold value, the methodcomprising: supplying, to the diode laser device as the control signal(202) and at a predetermined turn-on time, a bias signal having a valuewhich exceeds the threshold value, characterised by supplying to thediode laser device (203), as the control signal (202) and at apredefined time before the predetermined turn-on time, a pre-biassignal, which has a magnitude less than the threshold value and extendsfor a time period, the predefined time, magnitude, and time period ofthe pre-bias signal determining a required output power profile of theoutput optical signal (205).
 2. A method as claimed in claim 1, whereinthe pre-bias signal comprises a series of pre-bias pulses, havingrespective predefined times, magnitudes and extents, wherein thecombination of the plurality of pre-bias pulses causes the outputoptical signal to have the required power profile.
 3. A method asclaimed in claim 1, wherein the pre-bias signal is a stepped value.
 4. Amethod as claimed in any of claims 1 to 3, wherein the predeterminedturn-on time is defined by a clock signal.
 5. A method as claimed in anyof claims 1 to 4, wherein the predetermined turn-on time is determinedby a required output power profile of the output optical signal (205).6. A method as claimed in any of claims 1 to 5, wherein the value of thepre-bias signal is determined by a required output power profile of theoutput optical signal (205).
 7. A method of controlling a diode laserdevice (203) in an optical system (200), the system (200) including alaser diode device (203) a controller (201), wherein the laser diodedevice (203) is operable to receive a control signal (202) from thecontroller (201) and to output an optical signal (205) when the controlsignal (202) exceeds a threshold value, the method comprising:supplying, to the diode laser device as the control signal (202) and ata predetermined turn-on time, a bias signal having a value which exceedsthe threshold value, characterised by supplying to the diode laserdevice (203), as the control signal (202) and at a predefined timebefore the predetermined turn-on time, a pre-bias signal, which has amagnitude less than the threshold value and extends for a time period,the predefined time, magnitude, and time period of the pre-bias signaldetermining a required output power profile of the output optical signal(205).
 8. A method as claimed in claim 7, wherein the pre-bias signalcomprises a series of pre-bias pulses, having respective predefinedtimes, magnitudes and extents, wherein the combination of the pluralityof pre-bias pulses causes the output optical signal to have the requiredpower profile.
 9. A method as claimed in claim 7, wherein the pre-biassignal is a stepped value.
 10. A method as claimed in any of claims 7 to9, wherein the predetermined turn-on time is defined by a clock signal.11. A method as claimed in any of claims 7 to 10, wherein thepredetermined turn-on time is determined by a required output powerprofile of the output optical signal (205).
 12. A method as claimed inany of claims 7 to 11, wherein the value of the pre-bias signal isdetermined by a required output power profile of the output opticalsignal (205).
 13. An optical system (200) comprising, a controller (201)operable to output a control signal; and a laser diode device (203)operable to receive a control signal from the controller (201), and tooutput an optical signal (205) when the control signal (202) exceeds athreshold value, wherein the controller (201) is operable to output tothe laser diode device (203), as the control signal (202) and at apredetermined turn-on time, a bias signal having a value which exceedsthe threshold value to the laser diode device (203), characterised inthat the controller is operable to output to the laser diode device, asthe control signal (202) and before the predetermined turn-on time, apre-bias signal to the laser diode device (203), which pre-bias signalhas a magnitude less than the threshold value and extends for a timeperiod, the predefined time, magnitude, and time period of the pre-biassignal determining a required output power profile of the output opticalsignal (205).
 14. An optical system (200) as claimed in claim 13,wherein the controller (201) is operable to supply a pre-bias signalcomprising a series of pre-bias pulses, having respective predefinedtimes, magnitudes and extents, wherein the combination of the pluralityof pre-bias pulses causes the output optical signal to have the requiredpower profile.
 15. An optical system as claimed in claim 13, wherein thecontroller (201) is operable to supply a multi-valued pre-bias signal tothe laser diode device (203).
 16. An optical system as claimed in any ofclaims 13 to 15, wherein the controller is operable to output to thelaser diode device as the control signal (202) and before thepredetermined turn-on time, a pre-bias signal, which has a value lessthan the threshold value, and is defined by a clock signal of thesystem.
 17. An optical system as claimed in any of claims 13 to 16,wherein the controller is operable to output to the laser diode deviceas the control signal (202) and before the predetermined turn-on time, apre-bias signal which has a value less than the threshold value, whereinthe controller is operable to determine the predetermined turn-on timeby a required output power profile of the output optical signal (205).18. An optical system as claimed in any of claims 13 to 17, wherein thecontroller is operable to output to the laser diode device as thecontrol signal (202) and before the predetermined turn-on time, apre-bias signal which has a value less than the threshold value, whereinthe controller is operable to determine the value of the pre-bias signalby a required output power profile of the output optical signal (205).